Degree days are measures of how cold or warm it is at a certain location. A degree day compares the mean (the average of the high and low) outdoor temperatures recorded for that location to a standard temperature, usually 65° Fahrenheit (F). The more extreme the outside temperature, the higher the number of degree days. Degree days are an important measure that facilitate the normalization of climatic variation. In most commercial properties in the Southern United States, HVAC energy usage is typically the largest variable, so Degree Days are an essential aspect of understanding building energy consumption patterns.

Heating degree days (HDD) measure how cold the temperature was on a given day or over a period of days. For example, a day with a mean temperature of 40°F has 25 HDD.

Cooling degree days (CDD) measure how hot the temperature was on a given day or over a period of days. A day with a mean temperature of 80°F has 15 CDD. If the next day has a mean temperature of 82°F, it has 17 CDD. The total for the two days is 32 CDD.